Method of vulcanizing rubber and products thereof



Patented Jan. 15, 1952 METHOD OF VULCANIZING RUBBER AND rRon oTs THEREOF David J Beaver, St. Albans, W. Va., assignorto. Monsanto Chemical Company, St. Louis, Mo., a corporation of. Delaware 7 No Drawing.'

, This invention relates to the'vulcaniz'ation ofrubber. More particularly it relates to the vulcanization of rubber with anew class of accelerators and to the vulcanized products resulting thereby. w I

In the compounding of rubber, both natural and synthetic, improvements have been constantly sought, "particularly in respect to'the more eflicient and more economicaluse of 'v'ulcanization accelerators. *While many accelerators, per se, produce vulcanizates having acceptable physical properties, it' has long been ing accelerators possessing inherent weaknesses, V

such as inefiicient vulcanizing effect, limited solubility; and poor dispersibility in rubber stocks, thus causing uneven cures which in turn result in non-uniform properties "and erratic performance in service. It is also an. object of this invention to decrease the time of vulcanization. A further object is to provide a .wider choice of curing conditions such that a series of combinations varyingin scorch and ouringproperties may be obtained. For example, the ratio of activator to accelerator may vary depending upon the type of rubber, type of stock, tempera: ture of cure, time of cure, the particular ac: celerator, and the objective of the compounder, i. e., whether a low or high modulus is desired or a short or long cure is required. Other objects will appear hereinafter.

It has been found in accordance with this invention that theabove mentioned objects are 7 substantially attained bylvulcanizing a Vulcanizable rubber stock in which has been incorported a member of the group of compounds comprising amic acids, 011113111. amides ofrdicarboxylic acids, and their'salts of the following general formula ,nowise are they to such as a metal, ammonia, an amine, urea, and f Application January 17, 1949, Serial No. 71,390 M 18 Claims. (01. 260-798).

the like; where R1 and R2 are alkyl, aralkyl, and cycloalkyl groups such asmethyl, ethyl, ethanol, 2-cyanoethyl, p'ropyl, butyl, 2-cyanobutyl, amyl, benzyl,- phenethyl, cyclohexyl, methyl "cyclohexyl, and their various isomeric groups, car-" bamyl and hydrocarbon substituted-carbamyl radicals, thiocarbamyl and hydrocarbon substituted thiocarbamyl'radicals, and the like, and also either R1 or R2, but not both, may be hydrogen. The unsaturated group which links the two carbonyl carbon atoms may be part of an olefin'ic" nucleus, as "for example "in maleamicf acid *and in dimethyl maleamic acid, or -an" aromatic nucleus, as for example in phthalamic acid and in naphthalamic acid, and their substituted analogues and homologues. Amongthe salt forming groups are the alkaliand alkaline!- earth metals,'zinc, magesiumfand the like', animonia, methyl amine," ethyl amine, is'opropylf. amin'e, butyl amine, benzyl aminefcyclohexyl amine, dimethyl amine, diethyl amine, dipropyl amine, dibutyl amine, methyl propylflamine, N-methyl cyclohexylamine, dicyclohexylamine," dibenzyl amine, piperidine, trimethyl aniiflftri-i ethyl amine, and the like. I f t j Although amic acids and their salts of the above described general formula produce good results, the best results are obtained with phthalamic and maleamic acids, their metal salts, and their alkyl and cy'cloalkyl substituted amine salts, wherein at least one hydrogen atom of,the amido nitrogen isreplaced'by an alkyl or 1 amount group. As specific examples of this i'g roup are} N,N-dibutyl maleamic acid, N-cyclohexyl .malelamic acid, N,N-dicyclohexyl maleamic" acid, N ethyl phthalamic acid, N-butyl'phthalamic acid, N,N-diisopropyl phthalamic acid, N,N-dibutyl phthalamic acid, -N,N-dicyclohexyl phthalamic acid, zinc salt of N-butyl maleamic'acid, zinc salt of N-ethyl phthalamic acid, zinc salt of N-butyl phthalamic acid, zinc salt of N,N-d.icyclohexyl phthalamic acid, isopropyl amine salt of N-isopropyl maleamic acid, butyl amine saltof N-butyl maleamic acid, dicyclohexyl amine salt of N,N dicyclohexyl maleamic acidyethyl amine salt of N-ethyl phthalamic acid, isopropyl amine salt of N-isopropyl phthalamic acid, butyl"-- amine salt of N-butyl phthalamic acid, dicyclohexyl amine salt of N,N-dicyc1ohexyl phthalamic acid.

As illustrative of the preparation of the new activatorsv are the following examples ,but in be considered limitative thereof. l

' EXAMPLE'I v V 44.4 parts by weight (substantially 0.3115015 or phthalic anhydride and 108.8 parts by weight (substahtiallyjOfi mol) of dicyclohexylamine j mixed in a suitable reaction vessel and heated on a steam bath. A thick yellow syrupy liquid formed which on cooling gave 152.7 parts by weight (about 100% yield) of a yellow crystalline solid believed to be the dicyclohexyl amine salt of N,N-dicyclohexyl phthalamic acid,

7 having a melting point of 172 -174 o.

EXAMPLE 2' 90.1 parts by weight (substantially 2.0 mols'l of ethyl amine was distilled into a suitable rectotion vessel containing l l fparts by weight (substantially 1.0 mol) of phthalic anhydrideimnierjsed in a steam bath. A straw colored liquid developed which on separating. and drying. solidified to give 225 parts by weight (about 95% yield) of a crystalline product believed to be' the ethyl aminesaltof N -ethyl phthala nic acid,

oo-mcmc hydrochloric acid precipitating N,N-dicyclohexyl phthalamic acid,

which upon washing and drying was observed as having a melting: point of 189 C. Analysis; for

nitrogen gave approximately 4.4% (theoretical nitrogen 4.3%):

100 parts by weight (substantially 0.3. mol) of the above prepared- N,N-dicyclohexyl phthalamicacid was heated on a steam bath with 800 parts by weight of 21.25% potassium hydroxidesolution; A yellow oily; layer developed andwas separated.

Tothis yellow oily material was-added 40. parts by weight (substantially 0.3 mol) of-zinc-chloride.

A white gelatinous precipitate formed whichwas believed to be the zinc-salt of N;N-d icyclohexyl i phthalamic acid;

EXA PLE 14.4 parts, by weight (substantially 0.3 mol) of phthalic anhydride andx43z8 partsby weight. ('sub'v stantia' lly 0.6 mol). of butyl amine weretpl'ace'dfinja; Suitablereaction vessel" equipped with an agita' tor" 4 and a reflux condenser and heated over a steam bath for 15 minutes. A clear yellow liquid developedwhiclr on cooling gave-=37 parts by weight of a white: crystalline roduct having a melting point range of -110 C., believed to be the "butyl amine salt of N -butyl phthalamic acid.

EXALDPLE 6 To asuitable reaction vessel equipped with an agitator Qntaining 24.6 parts by weight (substantially mol); of maleic anhydride was added-fidfipartsby weight (substantially 0.5 mol) of butyl vamine over a period of thirty minutes at a; temperature below 15 C. Upon completion of tlieamine addition the temperature was raised to 1109' C. A pale yellow syrup developed which on cooling gave 61 parts by weight of a gelatinous light yellow colored liquid believed to, be the butyl amine salt of N-butyl maleami'c acid.

A miizt'ure containing" 148.1 parts by weight;

(substantially 1 mol). of phthalic anhydride and 76.2" parts by weight. (substantially 1 mol) of thiourea was, heated a suitable reaction vessel ata temperature of 1"4Q" 160'C. 218.8, parts" by weight of a yellow pasty material developed be lieved to be crude thiophtl'ialu'ric' acid. This. product was taken up with water and the solution made alkaline and filtered. Thereupon' the filtrate was made acid: with dilute: hydrochloric acid and filtered. The pale yellow residue was} dried and found to have a meltin 'point of 1 30 186'C'. anda nitrogen content of" 12.52 as com.- pared to a theoretical' n'itroge'n content of 12.50%.

As exemplary (if theactivating properties of the amic acids the following gum stocks" were compounded comprising The above st'o'cks socompounded' were vulcanize'd in the usual manner by heating a press 55 for difierenti periods 01 time at C. The physical propertiesot the respective cIi'r e'd rubber products are set forth below:

10 2,4 25 .2397. 2,767; inocurel 700% modulus,p.s.i 20 2119730. 1 2,457 1,885 mania-mama); s. il..{' 10 13%;? Ultimate elongation per- 1Q- 790 765 735 nocure. 70 cent 20 782 732 It is obvious from the above data that the amic acids and their. amine and metalsalts have ap- I preciably; accelerated the rated: cure of a natural.

5 rubber" stock.

As-further exemplary of this invention the following gum stocks were compounded comprising:

employed will vary outside the, range of the spe cific examples. For example as little as 0.2%.

E F G H I K L Parts by Parts by Parts by Parts by Parts by Parts by Parts by weight weight weight weight, weight weight weight Smoked sheets rubber 100. 100. 100. 100. 100. 100. 100. Zinc oxide 35. 35. 35. 85. 35. 35. 35. Sulfur 1.3 1. 3' 1.3 l. 3 1.3 1.3 1. 3 Stearic acid l. 1. 0 1. 0 1. 0 1. 0 1.0 1. 0 Benzothiazyl disulflde 0. 6 0. 6 0. 6 0. 6 0. 6 0. 6 0. 6 Ethyl amine salt of N-ethyl phthalamic acid 0! Exam e 2 1.0 Butyl amine salt of N-butyl phthaamic acid of Example 5 0. 8 O. 4 Butyl amine salt of N-butyl maleamic acid of Example 6 l. 0 0. 5 Thio phthaluric acid of Example 7. l. 0

The above stocks so compounded were vulcanlzed in the usual manner by heating in a press for various periods of time at 142 C. The physical properties of the respective cured products are set forth below:

on the rubberwas found to exert an activating efiect. Amounts higher than those specifically shown, as for example 2%, can be employed depending upon the conditions of compounding and the objectives of the .compounder.

Table II Min. Stocks cure at E F G H I K Y L 10 281 192 so 148 135 120 no cure. 300% i 325 22g 33 1 0 150 160 no cure. 10 884 84 0 1 8 630 478 no cure. 500% maul 2 gas 3 1 2 2 1 see no cure. 7 ,5 5 no cure.

20 3,595 353 2,850 35s. 10 3,980 .3 3 240 no cure. m 9

20 3,891 3,803 3,455 3:993 3:340 2, 623 491. Ultimate elmgatmn, Percent 53 323 33 325 353 313 3?? ie The data set forth in the above table show that the amic acid salts are a class of compounds which exert a marked activating effect on the cure rate and produce vulcanizates of high tensile strength in a short time. These new activators are readily compatible in rubber stocks, both natural and synthetic, and are nontoxic.

As exemplary of the activating properties of the amic acids in a synthetic rubber stock the following were compounded comprising The above stocks so compounded were vulcanized in the usual manner by heating in 'a press for different periods of time at 142 C. The physical properties of the respective cured rubber products are set forth below:

Table III Stock x i- 2 6 M O 2 1, 0 6 535 77 300% Mmmm' 1 48 1, a e 977 225 Ultimate elongation, per cent. 28 328 Ultimate tensile, p. s. i i8 3? Y The amic acids and their salts of themselves possess vulcanization accelerator properties. For.

example, the following natural rubber stocks were compounded comprising Parts by Parts by weight weight 100 100 5 5 3 3 Stearic acid l l Ethyl amine salt of N-ethyl phthalamic acid of Exam 9 2 .L 1.5 Dicyclohexylamine salt of N,N-dicyclohexyl phthalamic acid of Example 1 1. 5

- The above stocks so compounded were vulcanized in the usual manner by heating in a.

press for 60 and minutes respectively at 142 C.

pended claims is meant natural as well as synthetic rubbers which are capable of vulcanization when heated with sulfur and includes latices and reclaims of such materials whether or not admixed with'pigments, fillers, softeners. anti Obviously the amount of the new activators oxidants etc.

celeratorsor pr-imary accelerators are the mer- Ycaptothiazoles, mercaptoarylene thiazoles, thiand salts thereof, where R1 is selected from a group consisting of hydrogen, alkyl radicals of one to five carbon atoms inclusive and their monohydroxy and monocyano substituted derivatives, benzyl, phenethyl, cyclohexyl and methylcyclohexyl radicals, R2 is selected from a group consisting of alkyl radicals of one to five carbon atoms inclusive and their monohydroxy and monocyano substituted derivatives, benzyl, phenethyl, cyclohexyl and methy-lcyclohexyl radicals.

2. The method of vulcanizing rubber which comprises heating a rubber and sulfur in the presence of an amine 'salt'of an amic acid selected from the group consisting of phthalamic and maleamic acids, wherein there is attached to each nitrogen atom of the amic acid amine salt a radical selected from the group consisting of alkyl radicals of one to five carbon atoms inclusive and there monohydroxy and monocyano substituted derivatives, benzyl, phenethyl, cyclohexyl and methylcyclohexyl radicals.

3. The method of vulcanizing. rubber which comprises heating a rubber and sulfur in the presence of an amine salt of anamic acid selected from the group consisting of phthalamic and maleamic acids wherein there is attached to each nitrogen atom of the said amine salt an alkyl radical of one to five carbon atoms inclusive.

The method of vulcanizing rubber. which comprisesheat-ing a rubber and sulfur in the presence'of an amine salt of phthalamic acid wherein there is attached toeach nitrogen atom of the said amine salt an alkyl radical of one to five carbon atoms inclusive. I

5. The method of vulcanizing rubber which comprisesheating a rubber and sulfur in the presence of a metal salt of an amic acid selected from the group consisting of phthalamic and maleamicacids wherein there is attached to the amido nitrogen atom a radical selected from the group consisting of alkyl radicals of one to five carbon atoms inclusive and their monohydroxy and monocyano substituted derivatives, benzyl, phenethyl, cyclohexyl and methylcyclohexyl radicals.

c. The method 'of vulcanizing rubber whichcomprises heating rubber and sulfur in the presence of a thiazole accelerator and an amine salt of anamic acid selected from the group consisting "of phth'alamic and maleamic acids wherein there is attached to each nitrogen atoin' eiaeiapiapy "of ac'tivata'bIe for 'anic ac t of the said amine salt an "alkyl 'radical of o n e to five c'arbon atoms inclusive.

7. The method of vulcanizing rubber which comprises heating a rubber and sulfur in the presence 'of the 'butyl amine salt of N-butyl phthalamic acid.

8. The method of vulcanizing rubber which comprises heating a rubber and sulfur *injthie presence of the dicyclohexylamine salt of N',N- dicyclohexyl phthalamic acid. I

9. The method of vulcanizing rubber which comprises heating a rubber and sulfur in the presence of the ethyl amine salt of N-e'thyl phthalamic acid. 7 r I 10. The vulcanized rubber product obtained by heating a rubber andsulfur in the presence 'fof a compound selected from the-group consisting of acids of the structure and salts thereof, where R1 is selected from a group consisting of hydrogen, alkyl' radicals of one to five carbon atoms inclusive and their monohydroxy and monocyano substituted de- '11, The vulcanized rubber product obtained by heating a rubber and sulfur in the presence of anaiiiin'e salt of an amic acid selected from the group consisting of phthalamic and maleamicacids, wherein there is attached to each nitrogen atom of the amic acid amine salt a radical se lected from the group consisting of alkyl radicals of one to five carbon atoms inclusive and their monol'i'ydro'z'cy and monocyano substituted derivative's, benzyl, phenethyl, cyclohexyl and methylcycl'oheX'yl radicals.

12. The vulcanized rubber product obtained by heating a rubber and sulfur in the presence of an amine salt of an amic acid selected from the group consisting of phthalamic and maleamic acids wherein there is attached to each nitrogen atom of the said amine salt an alkyl radical 0 one to five carbon atoms inclusive.

13. The vulcanized rubber product obtained by heating a rubber and sulfur in-the presence of an amine salt of phthalamic acid wherein there is attached to each nitrogen atom of the said amine salt an alkyl radical of one to five carbon atoms l4=.' The'vulcanize'd rubber product obtained byheating a! rubber and sulfur in the presence of a metal salt of an amic acid selected from the group consisting of phthalamic and maleamic" amine salt an alkyl radical of one to five carbo atoms inclusive.

16. The vulcanized rubber product obtained by,

heating rubber and sulfur in the presence of the butyl amine salt of N-butyl phthalamic acid.

17. The vulcanized rubber product obtained by heating rubber and sulfur in the preseneje, 9f the dicyelohexyiamine salt of N,N-dicyc1o phthalaxr'iic acid.

18. The: vulcanized rubber product obtained by heating, rubber and sulfur in the presence 9f the ethyl amine salt of N-ethyl phthalamic acid.'

' DAVID J. BEAVER.

REFERENCES;

The following references arlof record in the file of this patent:

UNITED STATES. PATENTS Number Name Date 2,011,219 Harman Aug. 13, 1935 2,072,770 Reid Mar. 2, 1937 2,101,323 Salzberg Dec. 7, 1937 2,103,749 Horst Dec. 28, 1937 2,360,990

Valkenburgh Oct. 24, 1944 

1. THE METHOD OF VULCANIZING RUBBER WHICH COMPRISES HEATING A RUBBER AND SULFUR IN THE PRESENCE OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF ACIDS OF THE STRUCTURE 